1. Field of the Invention
The present invention is directed to a fluid-cooled X-ray tube, of the type wherein coolant flows in a closed cooling circulation path (loop) for the elimination of the generated heat.
2. Description of the Prior Art
In the case of x-ray tubes that, in particular, are provided for utilization in computed tomography systems, there is the desire or requirement to be able to eliminate the heat more efficiently directly from the tube. This desire exists particularly in associating with the need for performance enhancement of the tube, and affects glass bulb x-ray tubes as well as all-metal x-ray tubes, and rotating bulb tubes that are usually cooled with oil.
In glass bulb x-ray tubes, it is mainly the oil carbon deposits that arise at the anode-side glass bulb due to high local heating that have a catalytic influence on the further formation of oil carbon, resulting in the cooling becoming poorer locally in the advanced stages of the tube life, and the x-ray tube can then prematurely fail or the glass bulb can no longer be utilized for recycling due to the increased deposits of carbon residues.
In all-metal x-ray tubes, it is particularly the two smaller diameter passages (bottlenecks) at the cathode neck and the beam exit window that are subject to an especially pronounced heating. Here, as well, there is a greater need for cooling, particularly if it is desired to increase the short-term load of the tube. Due to the structural conditions, however, the cooling capacity cannot be increased without further measures, for example by installing a more powerful pump or by installing specific flow guidance members. The flow resistance would also be increased with the installation of flow guidance members, result in a rise in temperature of the coolant.
In rotating bulb tubes, the extremely high amount of heat at the anode cannot be eliminated rapidly enough by a direct transfer (heat flow) to the oil cooler that is usually present. The quantity of oil is usually limited due to space and weight reasons and therefore cannot be adapted to accommodate an increase in power and thus heat. In order to address this problem, attempts have already been made to install a specific intermediate store in the cooling circulation path so as to be able to intermediately store the heat that arises over the short term. Such an intermediate store, however, is a comparatively technologically complicated component, and the increase in weight associated with such a component leads to further problems due to the higher centrifugal forces in CT systems; and these problems have not been adequately solved. Providing an intermediate store also has the further disadvantage that the flow resistance for the oil flowing therethrough would rise and a more powerful oil pump therefore would have to be provided.
An object of the present invention is to provide a fluid-cooled x-ray tube of the type initially described wherein the cooling performance can be improved without having to accept the indicated disadvantages.
This object is achieved in accordance with the invention in a fluid-cooled x-ray tube wherein the cooling capacity is considerably increased by the employment of a coolant to which latent heat store elements in the form of micro-capsules are added, these co-circulating in the fluid stream of the coolant.
Elements referred to as latent heat storage elements are storage elements that contain a phase-change material, referred to in short below as PCM. Such PCM storage elements are characterized by the phase change material undergoing a phase conversion at a specific limit temperature. During this phase conversion, which ensues upon the application of energy, the temperature of the PCM remains practically constant, since the supplied energy is practically consumed for the phase change. The energy supplied during the phase conversion is thereby intermediately stored in the PCM storage elements and is in turn released upon reversal of the phase conversion. An increase in the temperature of the PCM occurs only after the phase conversion, given a further application of energy.
In the inventive employment, thus, the heat arising in the x-ray tube is intermediately stored in the PCM storage elements over a certain time span. Dependent on the selected material of the PCM and the amount of the PCM storage elements introduced into the coolant, the temperature of the coolant can be kept nearly constant over a specific time segment despite the heat arising in the generation of the x-rays. Compared to conventional measures for cooling an x-ray tube, the rise in temperature of the coolant is greatly retarded, so that the x-ray radiator can be more highly stressed (loaded) over the same operating duration, or the operating duration of the x-radiator can be significantly lengthened given the same load.
Primarily suitable as PCM materials for this purpose are paraffins whose melting temperatures lie between 90xc2x0 and 112xc2x0 C. A preferred paraffin PCM has, for example, a limit temperature of approximately 54xc2x0 C. at which the phase change occurs. As an alternative to paraffin, suitable fatty alcohols, fatty acids, hydrates of sodium carbonate, sodium acetate, calcium chloride and lithium magnesium nitrate also can be suitable.
The micro-capsules advantageously have a size of approximately 5 through 10 xcexcm, a maximum of approximately 20 through 50 xcexcm diameter, and are admixed to the coolant in a proportion of approximately 10 volume per percent. The body or the sheath of the capsules is advantageously composed of a polymerized carbon.
With the inventive measures, the heat capacity and thus the cooling capacity can be increased by a multiple. A particular advantage is that a faster elimination of the heat directly at the location at which it is created is achieved due to the constant flow of the PCM storage elements past the components generating the heat. The cooling of the components xe2x80x9con-sitexe2x80x9d thereby becomes far more efficient than without these PCM storage elements. Another advantage is that the flow-through quantity of the coolant need not be increased for enhancing the cooling capacity. The oil pump that is usually present therefore need not be dimensioned larger.